This invention relates to a method of evaluating a solid state image sensor by inspecting and judging electrical characteristics of a solid state image sensor.
Recently, solid state image sensors have been widely used in consumer-use and industrial fields. At the same time, there is a lot of demands for higher picture quality such as high resolution, wide dynamic range, low smear, and low dark current a high temperature.
Among solid state image sensors, especially in area sensors, the picture quality requirement is particularly severe, and it is important to inspect correctly and select products satisfying the picture quality specification determined between manufacturers and users.
Conventionally, the electrical charactersitics of a solid state image sensor were evaluated and sorted generally by testing the semiconductor wafer after the diffusion process by means of a tester. In this case, items of judgement are DC defects, extreme black or white flaws on the screen, and obvious contrast unevenness.
However, final judgement and ranking of the picture quality for satisfying the user's requirements are visually executed by a man. That is, a man checks a screen projected on the monitor television and inspects whether or not there are abnormal phenomena on the screen. The human visual sorting is a kind of a sensory inspection, that is, comparing the actual image with the standard image stored in an inspector's brain, and when the differences between them are within the allowable limit of the inspector, the product is regarded as conforming. In general, this sorting and comparing ability is excellent, and it is particularly effective for discovering tiny abnormal spots on an entire homogenous screen.
On the other hand, when evaluating solid state image sensors based on the electrical characteristics, the abnormality in DC operation or very clear black or white flaws on a screen are converted into values of voltage or current and such voltage or current values are measured. After that, when an item outside a standard specification is found, the product is handled as defective. However, in the case of white or black roughness of a delicate screen, a tiny black spot or white spot, or the background state of the screen at a high temperature in a dark screen, it is very dangerous to make a judgement based only a on voltage, current or other measurement. That is, when a screen which has slight defects is judged as defective, many products that may be approved by the visual impression of the screen may be judge as defectives. To the contrary, if only apparent defective products are rejected, there is a possiblility that multiple defective products may be mixed in with the conforming products.
Accordingly, there is no better way, at the present, than the method of actually operating a solid state image sensor, projecting the image on the monitor television screen, and judging the image visually by a man as mentioned above.
The human visual judgement, in spite of the excellent advantages as seen above, involves, among others, the following demerits.
(1) The criterion depends greatly on the individual ability of inspectors.
(2) The criterion is likely to be influenced by the individual mental state of an inspector, and it is difficult to always guarantee the same sorting level.
(3) A taining period is needed, and only limited inspectors may be engaged in the work in order to keep the inspection and selection level constant.
(4) Even a skilled inspector may sometimes make a mistake in judgement.
(5) It is difficult to increase the inspection speed.
In a representative solid state image sensor, hundreds of thousands of unit cells care neatly arranged, and each unit cell generates electrical charges depending on the quantity of light, and the generated electrical charges are sequentially transferred depending on the potential changes of the voltage. In this case, the reason why it is not possible to correctly select merely by the amplitude of the output value of each unit cell is that, for example, even if plenty of electrical charges are generated from a certain unit cell, when the surrounding unit cells also generate plenty of electrical charges, the impression of the screen is not necessarily bad for the human eye, or that, to the contrary, a unit cell suppresed at a very low electrical charge generation in a dark state may appear to be a white spot to the human eye on the screen having a lower background, so as to be judged as a defective screen.
Thus, the difficulty of inspection of solid state image sensors lies in that the relative impression of the entire screen to the human eye, black and white spots, roughness of the screen, and balance of whether disturbing to the human eye or not cannot be expressed by a mere electrical measurement.
Mostly, at the present, through the visual inspection, the impression of the screen is classified into ranks (for example, rank A, B, C and D), and the products are accordingly sorted and shipped.
Besides, when the technical analysis and investigation are needed, the data lacks objectiveness for discussion by the rank classification based on the impression of the inspectors, and the individual impressions themselves are also different.
Anyway, the demerits (1) to (5) of human visual inspection always occur as long as this method is employed.